An intermediate exchanger for a fast-neutron reactor transfers the heat from a stream of radioactive primary liquid sodium to a stream of non-radioactive secondary liquid sodium, while assuring physical separation between them. The primary liquid sodium is reheated by the core of the reactor and circulates in the vessel as far as the intermediate exchangers. The exchange of heat between the primary sodium and the secondary sodium is achieved by way of a bundle of tubes. Generally the secondary sodium circulates inside the tubes while the primary sodium circulates outside these tubes. The primary sodium and the secondary sodium circulate in contraflow. The circulation of the radioactive primary sodium outside the tubes facilitates the draining. Furthermore the secondary sodium operates at a higher pressure than that of the primary sodium and it is more economical to make the sodium at high pressure flow in the tubes. The tubes forming the tube bundle are generally vertical.
Different designs of intermediate sodium-sodium exchange for fast-neutron reactors are known. In accordance with one of these designs, the secondary sodium arrives above the tube bundle, the exchanger tubes of which are straight. This exchanger comprises a vertical inlet tube which is connected at its top end to the secondary sodium inlet. It channels a descending stream of relatively cold secondary sodium. This sodium is at a relatively high pressure (normally of the order of 6 bars, and exceptionally 18 bars in cases of accidental operation). This inlet tube is surrounded at its bottom portion by the tube bundle and it opens out below the bottom tube plate of this bundle. The secondary sodium leaving the inlet tube is confined in an inlet collector and rises in the exchanger tubes of the tube bundle. The secondary sodium is heated progressively by the primary sodium as it proceeds through the tubes of the tube bundle. The rising stream of reheated secondary sodium is channeled at the outlet from the tube bundle by an outlet collector extending vertically. This outlet collector consists of an inner tubular jacket and an outer tubular jacket which are coaxial and connected together and the top portion of the exchanger. These two jackets envelop the inlet tube for the secondary sodium. The outer jacket is provided with an outlet for discharge of the relatively hot secondary sodium.
The assembly formed by the tube bundle, the tube plates and the outlet collector is subjected to combined stresses due to forces from pressure and to forces from thermal dissymmetries in the tube bundle and in the outlet collector. The thermal forces are relatively large and are inherent in the design of the exchanger.
It has been observed that the exchanger tubes in the tube bundle have different average temperatures depending upon their positions with respect to the axis of symmetry of the bundle. This phenomenon tends to make the tube plates bend.
Furthermore it has been observed that a considerable temperature gradient exists in the secondary sodium flowing downstream of the upper tube plate of the tube bundle. The temperature increases as one goes horizontally from the inner jacket towards the outer jacket. This gradient causes considerable strains and deformations in the assembly immersed in the secondary sodium. The relative rigidity of the collector and of the tube bundle is such that the differential expansion of the inner jacket and of the outer jacket shows up as serious deformations of this assembly.